Oil and gas accumulations usually occur in porous and permeable underground rock formations. In order to produce the oil and gas contained in a formation, a well is drilled into the formation. The oil and gas, in many instances, are contained in the pore spaces of the formation, and are hydraulically connected by means of permeability or interconnecting channels between the pore spaces. After the well is drilled into the formation, oil and gas are displaced into the well bore by means of fluid expansion, natural and artificial fluid displacement, gravity drainage, etc. These various processes may work together or independently to drive the hydrocarbons into the well bore through existing flow channels. If production of the well is impaired by insufficient channels leading into the well bore, the formation, in many instances, is treated to increase the ability of the formation rock to conduct fluid into the well bore.
Acidizing of a subterranean formation penetrated by a well bore has been widely employed for increasing the production of fluids, e.g. crude oil, natural gas, etc., from the formation. The usual technique of acidizing a subterranean formation comprises introducing a nonoxidizing acid into the well under sufficient pressure to force the acid into the formation where it reacts with acid soluble components of the formation. The technique is not limited to formations of high acid solubility such as limestone, dolomite, etc. The technique is also applicable to other types of formations such as sandstone containing streaks or striations of acid soluble components such as ferrous carbonates.
During the acid treating operation, passageways for fluid flow are created in the formation or existing passageways therein are enlarged, thus stimulating the production of fluids from the formation. Acidizing operations wherein the acid is injected into the formation at a pressure or rate insufficient to create cracks or fractures in the formation is usually referred to as matrix acidizing.
Hydraulic fracturing is also widely used for stimulating petroleum producing subterranean formations and comprises the injection of a suitable fracturing fluid down a well bore penetrating a formation and into the formation under sufficient pressure to overcome the pressure exerted by the overburden. This results in creating a crack or fracture in the formation to provide a passageway which facilitates the flow of fluids through the formation and into the well. When the pressure of the fracturing fluid is reduced, a propping agent which is many times present in the fracturing fluid, prevents the complete closure of the fracture. Combination fracture-acidizing processes are well known in the art.
Increasing the viscosity of the fluid used in acidizing or fracturing of the formation has several benefits. In acidizing and/or fracture acidizing of subterranean formations, gelled fluids are useful in preventing the acid from becoming prematurely spent and inactive. In addition, gelling of the acid solutions enables the development of wider fractures so that the live acid can be forced further into the formation from the well bore. Furthermore, increasing the viscosities of the fluids permits better fluid loss control.
Water soluble polymers have been used in the past to increase the viscosity of fluids used in acidizing and fracturing processes.
Many water soluble polymers, however, exhibit one common problem which can be a difficulty to the user when it is desired to use the polymers in an aqueous solution. They are poorly dispersible in the water and thus require long periods of time to disperse in water as the individual particles tend to agglomerate when the polymer is mixed with water. Because rapid hydration takes place over the surface of the agglomerated aggregates to form gel-coated lumps and the interiors remain substantially dry, these lumps are then extremely difficult to disperse. This effect is aggravated by the fact that some polymers have a tendency to float on the surface of the water allowing partially dissolved particles to agglomerate into large lumps or masses. In order to break up the lumps or masses and facilitate rapid dispersion and solution of the polymers, it is necessary that the polymers be subjected to very strenuous agitation for relatively long periods of time.
One technique which has been proposed previously to prevent the agglomeration of water soluble polymers has been to treat the polymers with crosslinking agents, especially by crosslinking them with glyoxal. The crosslinking renders the polymers temporarily or partially insoluble to water. This method, however, has not been particularly effective on polymers which contain carboxyl groups and ionically charged polymers.
The present invention provides a method of treating water soluble anionic polymers which overcomes or at least mitigates the above-mentioned problems.